The invention relates generally to dose monitor circuitry, and more specifically, it relates to a means of measuring the total ionizing radiation dose using silicon solid state devices.
Past implementations of total ionizing radiation dose measurement circuits have relied on Geiger-Mueller tubes, thermoluminescent devices (TLD's), and/or single silicon detector devices. Geiger-Mueller tubes and TLD's have many disadvantages not necessary to discuss here, Detectors using silicon devices such as diodes and P-channel metal oxide semiconductor (PMOS) transistors have been implemented in the past. Diode detectors are typically insensitive to total ionizing radiation dose at levels of interest for characterizing semiconductor degradation. Thus, a diode detector is not usually capable of accurately measuring low total dose levels with good resolution. PMOS transistor implementations of total ionizing radiation dose measurement circuits are typically encountered in applications requiring nigh accuracy, low total dose measurement. Past art used PMOS transistors for the sensor element and biased the transistor using a constant current source tied to the source of the transistor and connected the gate and the drain of the transistor to ground. By measuring the voltage appearing at the source input of the transistor, a direct reading of the gate threshold voltage could be made. The problem with this implementation of the total dose sensing element is three-fold.
First, the PMOS transistors bias voltage is changing dynamically as it is exposed to ionizing radiation. This is an undesirable condition because the total dose response of a PMOS transistor to ionizing radiation exposure is linear only if the electric field across the device remains constant. This obviously is not the case with past art.
The second deficiency in past art is the reliance on a constant current source to bias the PMOS transistor. Constant current sources used in past art typically degrade as a function of total ionizing radiation dose exposure. This makes the constant current vary with dose exposure and invalidates any calibration performed on the PMOS transistor for total dose response at the nominal constant current set-point.
The third deficiency in past art is the dependence in the measurement output on the load appearing at the source terminal of the PMOS transistor. Any load appearing at the source of the transistor will; a) vaiy the current going into the transistor because some current is shunted into the load, and b) as a result the electric field across the device is not constant.
The task of providing dose monitor circuitry is alleviated by the following U.S. Patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 3,614,444 issued to Nirshl; PA1 U.S. Pat. No. 3,654,468 issued to Shah; PA1 U.S. Pat. No. 4,983,840 issued to Ouvrier-Buffet et al; and PA1 U.S. Pat. No. 5,309,085 issued to Sohn.
The four references disclose alternative measuring circuits that are improved by the present invention.